The present invention relates to molds for tires and more particularly molds of the sector type. In this type of mold, the molding space for the outer surface of the tire is defined by two shells, each assuring the molding of a side wall, and by a ring of sectors assuring the molding of the tread.
The manufacture of tires, and more particularly the vulcanization phase, requires placing the tire blank under pressure and supplying heat to it.
The "sector" type curing molds are made of several parts assembled and operated by a suitable kinematic system which is well known to those skilled in the art. One example thereof is given in U.S. Pat. No. 3,797,979.
During the vulcanization, a certain pressure is exerted on the material to be molded (raw rubber), which, furthermore, experiences a substantial decrease in viscosity produced by the increase in temperature. Ideally, in order to avoid mold burrs, the joint planes of the curing mold should remain tight or have a clearance less than or equal to the small amount of clearance permitted by the physical properties of the rubber mixes at the time of the vulcanization (a few one-hundredths of a millimeter, on the order of 0.03 mm). The term "joint planes", as used herein, refers to the interfaces between each integral unit constituting the mold, namely the shells and the sectors.
Unfortunately, this tightness is generally insufficient in actual practice and molding defects occur, known as "molding burrs", formed of rubber which has worked its way into the interfaces present between sectors and between shells and sectors.
This problem in the present state of the art is illustrated in FIGS. 1 to 3 of the accompanying drawings, in which FIG. 1 is a section through a meridian plane of a known sector mold, showing a sector 1 and the side shells 2, and FIGS. 2 and 3 are a section through the plane I of FIG. 1 perpendicular to the axis of the mold, diagrammatically indicating two types of configurations which these sector molds have in actual practice.
The present design of the molds can be compared with a cylinder which is split into sectors which are subjected at the same time to internal and external pressures. We will refer to the imaginary circle drawn at the interface with a side shell 2 measured below the ring of sectors 1 when all the sectors 1 are in contact with each other in the circumferential direction as the "embedment circle" C.sub.1. The corresponding circle measured on the shell 2 will be referred to as the shell circle C.sub.2. The expression "circle" is intentionally used, since this interface is either cylindrical or frustoconical, as in the figures. Technically, one can speak of a circle only for one given axial position, but this technical detail is without importance for an understanding of the problem raised and the solution contributed by the invention.
In the case of FIG. 2, the embedment circle C.sub.1 is larger than the shell circle C.sub.2. Upon the closing of the mold, there is no play between sectors 1. Radial play J.sub.1 is present between the embedment circle C.sub.1 drawn on the ring of sectors 1 and the shell circle C.sub.2.
In the case of FIG. 3, the sector embedment circle C.sub.1 which could have been measured if the sectors could have been brought into contact with each other would be shorter than the shell circle C.sub.2. This movement together is impossible, since the sectors 1 first come into contact with the shells. A clearance J.sub.2 remains between the sectors 1 abutting against the shell. There is, therefore, no clearance between a sector 1 and the shell 2. It is only in the theoretical intermediate case contained between the configuration illustrated in FIG. 2 and that illustrated in FIG. 3 that one could have total tightness of the mold.